Chemical feeders are commonly used throughout the water treatment industry for insertion of a concentrated solution to treat a particular attribute. For instance, a chelant may be used to control scaling by forming heat stable soluble complexes with calcium and magnesium; sodium sulfite may be used to prevent oxygen corrosion; polymers may be used to disperse sludge or aid in the removal of undesirable particles.
Most water treatment chemicals are inserted in a concentrated form by use of a liquid chemical feeder. The chemical feeder injects a predetermined amount of the concentrated chemical from a solution tank. The solution tank may consist of a premixed solution or be used as a make-up tank where a dry powder is admixed with water to form the concentrated solution. For explanation purposes only, the focus of this application is directed to polymers which are tightly tangled structures before activation and become untangled and activated upon proper dilution with water.
One such polymer, known as polyelectrolytes, are high molecular-weight polymers used for, among other things, an aid in removing suspended particles from water, for dewatering sludges, and for other liquid-solid separation applications. This polymer is typical of dry powder chemicals wherein proper mixing is required to cause activation without damaging of the chemical structure.
The use of premixed polymers are relatively expensive. Premixed polymers are shipped as a concentrated liquid in drums, the drums are difficult to handle and liquid has a limited shelf life. Depending upon the type of polymer, additional mixing to maintain the polymer in solution or transfer to a solution tank having a greater dilution may be necessary. Spillage of a liquid polymer presents a hazardous situation for liquid polymer creates an extremely slippery surface.
Dry polymer is desirable for many applications because it can be easily stored and shipped. A dry powder is relatively light and if spilled can be easily recovered. Dry polymers are permissible for certain food grade and potable applications, whereas premixed liquid polymers are not, as it is difficult to control bacteria in a liquid carrier. The problem with dry polymers is the need for mixing with water before use.
Typically dry powder is placed into a solution tank and diluted with water where it is mixed by hand or by use of an electric mixer. Dry polymers are hygroscopic and its suspension in water is thixotropic. For this reason, known systems for purposes of mixing dry polymers with water are subject to agglomeration of dry polymer particles during the wetting procedure. For this reason, a dry polymer must be mixed correctly or improper concentrations will be formed causing difficulty in the water treatment process as well as feeder problems. Point of use mixing frequently fails to properly mix dry polymers which require a shear mix that is sufficient to cause proper wetting but not high enough to damage the polymer structure. Further, the polymer must be maintained in solution requiring constant mixing, again without damaging the polymer structure.
The assignee has developed many devices in this area to address this situation. For instance, U.S. Pat. No. 4,778,280 discloses a device for mixing dry polymer with a liquid. The device consists of a pump having a centrifugal impeller with a means for inserting a predetermined amount of polymer and liquid into the suction side of the impeller allowing the impeller to admix the solution. U.S. Pat. No. 5,018,871 discloses a polymer dilution and activation apparatus also having a polymer and water intake placed adjacent to the pump impeller allowing instantaneous mixing through high shear forces which discourages the polymer from forming gel aggregates.
U.S. Pat. No. 5,135,968 provides a primary dilution and activation apparatus having a predefined chamber capable of mixing the polymer and liquid in a processing zone which subjects the polymer to relatively high shear conditions for a relatively short period of time in one zone, and subjects the solution to a continuously decreasing shear rate for a longer period of time in the second zone.
What is lacking in the art is a low maintenance automatic dry powder batch activation system for the mixing of dry powder into a concentrated solution for liquified insertion into a water treatment system.